BOSTON -- Aaron Hernandez's death in prison has been ruled a suicide, and the former NFL star's brain is being donated to sports concussion researchers, Massachusetts authorities said Thursday.

The declaration by prosecutors, state police and public health officials came after a tumultuous day in which Hernandez's lawyer suggested the state was mishandling the investigation and illegally withholding his brain after releasing the rest of the body to a funeral home.

Authorities said the medical examiner had ruled that the cause of death was asphyxia by hanging and that investigators had found three handwritten notes next to a Bible in Hernandez's cell at the Souza-Baranowski Correctional Center in Shirley. Authorities previously said Hernandez had not left a suicide note and he hadn't been on suicide watch.

The mother of the man who former Patriots star Aaron Hernandez was convicted of killing says she is convinced justice was served even if the murder charge gets erased.

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"There were no signs of a struggle, and investigators determined that Mr. Hernandez was alone at the time of the hanging," the statement read.

Hernandez had been locked into his cell at about 8 p.m. Tuesday and no one entered the cell until a guard saw him just after 3 a.m. Wednesday and forced his way in because cardboard had been jammed into the door track to impede entry, authorities said. Hernandez was found hanging from a bedsheet and rushed to a hospital, where he was pronounced dead about an hour later.

Earlier Thursday, Hernandez's lawyer argued that state officials had turned over the 27-year-old's body but not his brain.

Attorney Jose Baez said the family had arranged for researchers at Boston University's Chronic Traumatic Encephalopathy Center to take custody of the brain. Now that the effort to confirm Hernandez took his own life is not in question, officials say the brain was released to the center in accordance with the wishes of Hernandez's family. The center studies a progressive degenerative brain disease found in some athletes who have experienced repetitive brain trauma.

Hernandez's body is at a Boston-area funeral home, but services likely will be held elsewhere for the Connecticut native.

Baez said he has retained Dr. Michael Baden, a former chief medical examiner for New York City, to perform an independent autopsy.

Authorities in Massachusetts say Aaron Hernandez's brain is being turned over to university researchers now that they've ruled his death in prison a suicide. Nancy Lane/The Boston Herald via AP

Baden, who didn't immediately comment, has performed autopsies in several high-profile cases, including the death of Michael Brown, a black teen who was shot and killed by a white police officer in Ferguson, Missouri, in 2014.

Baez declined to say whether he or the family believed brain damage from Hernandez's playing days led the 27-year-old former New England Patriots player to kill himself.

"We're not suggesting anything," he said. "You go where the evidence takes you. We need to examine every aspect of this case."

It's generally best for researchers to get access to a brain within hours of death to determine the presence of chronic traumatic encephalopathy or other neurodegenerative diseases, said Dr. Lee Goldstein, a CTE researcher at Boston University. Researchers also prefer to receive the entire brain, as a small piece may not tell the whole story, he added.

Other questions surrounding Hernandez's death still remain unanswered.

Authorities still have not released the incident report, officers' logs or video footage from the area around Hernandez's cell despite repeated requests from The Associated Press.

Hernandez's fiancee, Shayanna Jenkins Hernandez, filed an order Wednesday on behalf of the daughter she had with Hernandez, asking that prison officials be barred from altering or destroying any potential evidence, including Hernandez's writings, video and audio recordings and medical records.

A judge in New Bedford is due to hear the request at 2 p.m. Friday.

Massachusetts Gov. Charlie Baker said he wasn't drawing any conclusions until the full details of the investigation are revealed.

"Anytime someone kills themselves in prison, something clearly went wrong,'' he said, adding that he had full confidence in prison officials and was unaware of any staff being reprimanded.

Another unknown: why Hernandez would kill himself just days after he was cleared of two murder charges.

He had been serving a life sentence without parole for the 2013 slaying of a onetime friend. But during his trial in Boston for the killing of two men in Boston in 2012, he appeared upbeat, constantly backslapping his lawyers, letting out bellowing laughs and blowing kisses to his 4-year-old daughter and other family members in the audience.

The former University of Florida standout died five days after a jury acquitted him in those two deaths, which prosecutors alleged were precipitated by one of the men accidentally spilling a drink on Hernandez at a Boston nightclub.

His death also came just hours before his former New England Patriots teammates visited the White House on Wednesday to celebrate their Super Bowl victory.

Hernandez's death left friends, family and his legal team in disbelief as many searched for an explanation to the tragic end of a young man whose football skills at one point earned him a five-year, $40 million contract extension with the NFL's top franchise.

Baez on Thursday said the family was in mourning, but he declined to comment on a range of new reports, including that Hernandez was on the phone with Jenkins-Hernandez hours before he was found hanging in his cell early Wednesday. That was first reported by The Boston Globe, citing another one of Hernandez's lawyers.

Hernandez's older brother, Jonathan "DJ'' Hernandez, meanwhile, took to Facebook on Thursday to wish his mother, Terri Hernandez, happy birthday. "I know if Aaron was here one more day he would have said he loves you,'' he wrote. "Keep smiling because I know Aaron and Dad are both smiling down on you [and] us right now.''